Electroplating OR Electrodeposition on Machine Tools

Electroplating OR Electrodeposition on Machine Tools

Electroplating OR Electrodeposition on Machine Tools - Electroplating deposits an adherent coating of metal on a surface of an object.

Metals deposited are
Cadmium - Chromium
Copper  - Nickel
Silver - Tin
Zinc and which may be deposited on the same base metal or on a different metal or on non-metal (plastic parts).

Alloys such as
Brass - Bronze
Solder – Cu-Zn
Ni-Mo – W-Co, etc are also electrodeposited.

Electroplating is a process in which a direct current is passed between an anode (+) and a cathode (-), the two being immersed in a plating tank containing electrolyte solution (metallic salt solution).

The workpiece to be plated is made the cathode and the material to be deposited is made the anode. When an electric current is passed from the anode to the workpiece to be plated through the medium of an electrolyte, the anode transfers some of its mass to the bath (of electrolyte) and an equal amount of the material is deposited on the cathode which is the workpiece to be plated.

Copper Plating Cell

Electroplating with nickel or chromium is now commonly used for building up on worn surfaces, and for depositing a hard surface to give more efficient wearing properties. We might give a short resume of the principle of electroplating taking Copper as an examle.

If a platee of pure copper is connected to a + ve D.C. electric supply and immersed in a solution of a copper salt (e.g. the sulphate, CUSO₄) whilst another plate of a suitable metal (e.g. steel) is also immersed and connected to the -ve of the supply, when a current is passed the electric potential between the two plates dissociates the CuSO₄ into positive copper (Cu) ions and negative SO₄ ions.

The copper ions being positive are attracted to the -ve plate (cathode) and are deposited as metal, whilst the SO₄ ions go to the +ve copper plate (anode) where they attack the copper, the action resulting in CUSO₄ being formed. This formation of CuSO₄ at the anode replaces that remove from the solution by the electrolytic action, so that as the process goes on, copper is lost from the anode and deposited on the cathode .

The electrolyte used for chromium plating is chromic acid (CrO₃) and for nickel deposition the solution is made up by mixing various nickel salts with nickel ammonium sulphate and nickel sulphate as the chief constituents.

The equipment necessary to carry out the process consists of a motor generator, or transformer and rectifier (for A.C. supply) to provide the current, electrical control gear, deposition vat together with an exhaust system, cleaning tanks, washing tanks, drying equipment, etc.

The preliminary process to plating is a thorough cleaning of the article, since scrupulous chemical cleanliness, is essential for success. This may be conducted by acid or electrolytic methods after preliminary degreasing for dirty components.

The components are then suspended as cathodes in the plating vat and current passed for sufficient time to deposit the required thickness of metal. Average rates are about 0.001 inch per hour for nickel and half this for chromium, but these vary according to the shape of the surface being treated.

Heavy currents are necessitated for plating, but the voltage is low. For chromium the current necessary is of the order of 75 to 100 amps. Per sq. ft. of area deposited with a pressure of about 6 volts.

To prevent reactions that would be detrimental to the adhesion of the deposited metal it is usual to plate a thin layer of one metal before adding the proper coating of the metal required for example, it is usual, before chromium plating, to deposit a layer of nickel about 0.001 in. thick.

The hardness of chromium as deposited varies between 750 and 950 Brinell, so that it may only be machined by grinding. Nickel deposits vary between 250 and 450 Brinell and may, in general, be machined with cutting tools. Theoretically there is no limit to the thickness of metal that may be deposited, but in practice the economic limit is about 0.05 to 0.1 in. of nickel and 0.01 in of chromium.

The uniformity of the deposit will be influenced by the profile of the surface and the metal tends to crowd on to projecting comers, and this characteristic must be taken into account if there is to be subsequent machining to ensure that there will be a sufficient thickness of deposited metal to clean up.

Excess At Projecting Corners. Deficiency At Root of Angle	Excess At Corners. Deficiency At Bottom of Slot

Effect of Surface Irregularities on Thickness of Deposited Metal

A - Deposited Layer

Normally, when metal has been deposited, subsequent machining is employed to restore the surface back to the desired sizes. For nickel this may be done by cutting in the same way as steel and/or grinding, but chromium may only be ground.

For surfaces having simple shapes on which a uniform thickness of deposit can be assured (e.g. shafts, flats, etc.), deposits up to 0.005 in thick can be easily controlled to a tolerance of :t 0.001 in and less, so that provided the base metal is smooth and accurate the plated surface may often be put into service without any further treatment.

The extreme hardness of chromium has led to the wide adoption of chromium plating on surfaces subject to wear and abrasion, and the process may be applied either to reclaim worn surfaces, or plating may be put on in the first instance. Examples of this occurs in the following: plug and gap gauges, drills and reamers, milling cutters, dies used for plastic moulding, and so on.

Chromium plated gauges with a deposit of 0.001-0.003 in. will give many times the life of similar gauges in hardened steel. An example showing the reclamation of a splined shaft by electro-deposition is (1) Worn, (2) After metal deposition (3) Machined. Chromium plating is widely used for the restoration of the working surfaces of the shafts, spindles etc., where wear is up to 0.24 mm.